The present invention relates to an irradiation system and method, and more particularly to an x-ray irradiation system configured to uniformly expose large articles such as loaded shipping pallets to ionizing radiation.
Irradiation systems for pasteurization of food products and materials increasingly employ machine-generated radiation as the source of the ionizing radiation that eliminates harmful food borne pathogens. The FDA and USDA approved machine generated radiation sources are high energy accelerated electrons and x-ray photons. Electron beam irradiation is a process in which high-energy electrons up to 10 MeV are directed toward food products to be processed. Because of their particle characteristics, high-energy electrons have limited penetration capability and are useful for irradiating materials only up to about 4 centimeters (cm) in thickness in single-sided configurations and about 9 cm in two-sided applications.
By contrast, x-rays are capable of much greater penetration. The x-ray depth-dose curve is characterized by an e-x mathematical function with a “tail” that penetrates on an attenuated but continuing basis. The e-x curve represents both an opportunity and a difficulty for irradiation of large articles. The opportunity is afforded by the deep penetration potential of the depth-dose curve tail. The difficulty is that it is important for the delivered radiation dosage in food products to be as uniform and constant as possible. An important measure of dosage uniformity is the maximum/minimum ratio defined as the ratio between the peak dose and the minimum dose over the physical volume of the product being processed. Ideally, the max/min ratio would be as close to 1.0:1 as possible. In general, a max/min ratio of 1.3:1 to 1.6:1 is considered acceptable for most applications.
Two-sided application of x-ray radiation is somewhat helpful to improve the max/min ratio for materials up to about 45 cm (18 inches) in thickness. Unfortunately, there are many food material configurations that require processing of thicknesses up to 48 inches, in particular products that are packaged in boxes and placed on palleted shipping containers. The typical two-sided penetration capability of the available radiation sources is insufficient to deliver a max/min ratio as low as 1.6:1 for materials this thick. An alternate methodology is needed to provide this capability.